U.S. patent number 8,294,267 [Application Number 10/588,369] was granted by the patent office on 2012-10-23 for nanostructures and method for selective preparation.
This patent grant is currently assigned to Yissum Research Development Company of the Hebrew University of Jerusalem. Invention is credited to Uri Banin, Taleb Mokari.
United States Patent |
8,294,267 |
Banin , et al. |
October 23, 2012 |
Nanostructures and method for selective preparation
Abstract
The present invention provides novel nanostructure composed of
at least one elongated structure element, an elongated structure
element of said nanostructure bearing a different zone made of
metal, metal alloy, conductive polymer or semiconductor and
selectively grown onto at least one of the end portions of the
elongated structure element. The present invention further provides
a selective method for forming in a liquid medium, such
nanostructures.
Inventors: |
Banin; Uri (Mevasseret Zion,
IL), Mokari; Taleb (Jerusalem, IL) |
Assignee: |
Yissum Research Development Company
of the Hebrew University of Jerusalem (Jerusalem,
IL)
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Family
ID: |
34812092 |
Appl.
No.: |
10/588,369 |
Filed: |
February 3, 2005 |
PCT
Filed: |
February 03, 2005 |
PCT No.: |
PCT/IL2005/000133 |
371(c)(1),(2),(4) Date: |
October 16, 2007 |
PCT
Pub. No.: |
WO2005/075339 |
PCT
Pub. Date: |
August 18, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080128761 A1 |
Jun 5, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60541248 |
Feb 4, 2004 |
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60554913 |
Mar 22, 2004 |
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Current U.S.
Class: |
257/746; 257/734;
438/497; 257/741; 257/E29.345; 977/762 |
Current CPC
Class: |
H01L
29/0673 (20130101); B82Y 30/00 (20130101); H01L
29/0665 (20130101); C30B 29/605 (20130101); B82Y
10/00 (20130101); H01L 29/0669 (20130101); Y10T
428/24917 (20150115); H01L 2924/0002 (20130101); H01L
2924/0002 (20130101); H01L 2924/00 (20130101) |
Current International
Class: |
H01L
23/48 (20060101); B32B 15/00 (20060101) |
Field of
Search: |
;257/288,9,734,741,746,E29.345 ;438/497 ;977/762 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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9106036 |
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May 1991 |
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WO |
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0229140 |
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Apr 2002 |
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WO |
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02079514 |
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Oct 2002 |
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WO |
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03054953 |
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Jul 2003 |
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WO |
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03091458 |
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Nov 2003 |
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WO |
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03097904 |
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Nov 2003 |
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WO |
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WO 03091458 |
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Nov 2003 |
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WO |
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WO 03097904 |
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Nov 2003 |
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WO |
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Primary Examiner: Richards; N Drew
Assistant Examiner: Dulka; John P
Attorney, Agent or Firm: Browdy and Neimark, PLLC
Claims
The invention claimed is:
1. A nanostructure having a branched shape with at least two
elongated structure elements of a first material, wherein each of
said elongated structure elements has an end portion, and wherein
at least one of said end portions bears a nanozone of a second
material that differs from said first material in at least one
property selected from the group consisting of electrical
conductivity, chemical reactivity and composition.
2. The nanostructure according to claim 1, wherein said second
material is a metal or metal alloy.
3. The nanostructure according to claim 1, wherein said second
material is a conductive polymer or an insulating material.
4. The nanostructure according to claim 1, wherein said second
material is a semiconductor material.
5. The nanostructure according to claim 1, wherein said first and
second materials are each a semiconductor material selected from
the group consisting of Group II-VI semiconductors, Group III-V
semiconductors, Group IV-VI semiconductors, Group IV
semiconductors, alloys made of these semiconductors, combinations
of the semiconductors in composite structures and core/shell
structures of the above semiconductors.
6. The nanostructure according to claim 5, wherein said Group II-VI
semiconductors are alloys selected from the group consisting of
CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, and combinations thereof.
7. The nanostructure according to claim 1, wherein said first
material is CdSe or CdSe/ZnS in a core/shell layered arrangement
and said second material is gold.
8. A self assembled construct, comprising a plurality of
nanostructures according to claim 1, wherein each nanostructure in
the construct is linked to another nanostructure in the construct
through the nanozones on the end portions of elongated structure
elements thereof.
9. A solution comprising a plurality of nanostructures according to
claim 1.
10. The nanostructure according to claim 1, wherein each of the end
portions of said elongated structure is coupled to a nanozone.
11. The nanostructure according to claim 1, being selected from the
group consisting of a tripod and a tetrapod.
12. The nanostructure according to claim 1, wherein said first
material is selected from the group consisting of a semiconductor
material, an insulating material, a metal and a combination
thereof.
13. The nanostructure according to claim 1, in the shape of a
branched bipod.
14. A method for forming a nanostructure having at least one
elongated structure element of a first material, each said
elongated structure element having an end portion, and a nanozone
of a second material on the end portion of at least one of the
elongated structure elements, said first and second materials being
different in at least one property selected from the group
consisting of electrical conductivity, chemical reactivity and
composition, said method comprising: providing a solution of
nanostructures consisting of a first material, each nanostructure
having at least one elongated structure element having an end
portion; contacting said nanostructures in solution with an agent
of a second material, said agent being selected from the group
consisting of a metal source, a metal alloy source, a conductive
polymer source, an insulating material source and a semiconductor
source; and allowing growth of said at least one agent of a second
material on the end portion of at least one of the elongated
structure elements of said nanostructures, to thereby obtain
nanostructures bearing at least one nanozone on the end portion of
at least one of the elongated structure elements.
15. The method according to claim 14, wherein said agent is
selected from the group consisting of a metal source and a metal
alloy source.
16. The method according to claim 14, wherein said first material
is selected from the group consisting of a semiconductor material,
an insulating material, a metal and a combination thereof.
17. The method according to claim 16, wherein said first material
is a semiconductor material.
18. The method according to claim 17, wherein said nanostructure is
selected from the group consisting of a bipod, a tripod and a
tetrapod.
19. A method for forming an electrically conductive zone on a
nanostructure having at least one elongated structure element, said
method comprising: providing an organic solution of semiconductor
nanostructures consisting of a first material, each nanostructure
having at least one elongated structure element having an end
portion; contacting said nanostructure in said organic solution
with another organic solution comprising a metal or metal alloy
source, a stabilizer and/or a surfactant and/or electron donor; and
allowing growth of said metal or metal alloy on the end portion of
at least one of the elongated structure elements of said
semiconductor nanostructures, to thereby obtain semiconductor
nanostructures, bearing at least one electrically conductive
nanozone of metal or metal alloy on the end portion of at least one
of the elongated structure elements.
20. The method according to claim 19, wherein said nanostructure is
in a form selected from the group consisting of a nanorod, a bipod,
a tripod, a tetrapod, a nanowire and a nanotube.
21. A nanostructure having at least one elongated structure element
of a first material, wherein each of said elongated structure
elements has an end portion, and wherein each of said end portion
is coupled to a nanozone of a second material that differs from
said first material in at least one property selected from the
group consisting of electrical conductivity, chemical reactivity
and composition, wherein at least one of the first and second
materials is a semiconductor material, and wherein the second
material is in direct contact with the first material.
22. The nanostructure according to claim 21, wherein said first
material is a semiconductor material and said second material is a
metal or metal alloy.
23. The nanostructure according to claim 21, wherein said second
material is a semiconductor material.
24. The nanostructure according to claim 21, wherein said first and
second materials are each a semiconductor material selected from
the group consisting of Group II-VI semiconductors, Group III-V
semiconductors, Group IV-VI semiconductors, Group IV
semiconductors, alloys made of these semiconductors, combinations
of the semiconductors in composite structures and core/shell
structures of the above semiconductors.
25. The nanostructure according to claim 24, wherein said Group
II-VI semiconductors are alloys selected from the group consisting
of CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, and combinations thereof.
26. A self assembled construct, comprising a plurality of
nanostructures according to claim 21, wherein each nanostructure in
the construct is linked to another nanostructure in the construct
through the nanozones on the end portions of elongated structure
elements thereof.
27. A solution comprising a plurality of nanostructures according
to claim 21.
28. The nanostructure according to claim 21, being selected from
the group consisting of a bipod, a tripod and a tetrapod.
Description
FIELD OF THE INVENTION
This invention relates to the field of nanomaterials.
LIST OF REFERENCES
The following references are considered to be pertinent for the
purpose of understanding the background of the present invention:
1. M. S. Gudiksen, L. J. Lauhon, J. Wang, D. Smith, and C. M.
Lieber, Nature 415, 617 (2002). 2. Y. Wu, R. Fan, P. Yang, Nano
Lett. 2, 83 (2002). 3. D. V. Talapin, R. Koeppe, S. Goltzinger, A.
Kornowski, J. M. Lupton, A. L. Rogach, O. Benson, J. Feldmann, and
H. Weller, Nano Lett. 3, 1677 (2003). 4. WO 03/097904 5. WO
03/054953 6. Y. Cui and C. M. Lieber, Science 291, 851 (2001). 7.
S. Heinze, J. Tersoff, R. Martel, V. Derycke, J. Appenzeller, and
Ph. Avouris, Phys. Rev. Lett. 89, 106801 (2002). 8. A. Javey, J.
Guo, Q. Wang, M. Lundstrom and H. Dai, Nature 424, 654 (2003). 9.
Z. A. Peng, X. Peng, J. Am. Chem. Soc. 123, 1389 (2001). 10. J. E.
Cretier and G. A. Wiegers, Mat. Res. Bull. 8, 1427 (1973). 11. W.
W. Yu, Y. A. Wang, X. Peng, Chem. Mater. 15, 4300 (2003). 12. D.
Coucouvanis, Prog. Inorg. Chem. 11, 233 (1970); 13. Jun Y.W.; Lee
S. M.; Kang N. J.; Cheon J.; J. Am. Chem. Soc. 123, 5150 (2001).
14. U.S. Pat. No. 5,505,928. 15. L. Manna, D. J. Milliron, A.
Meisel, E. C. Scher, A. P. Alivisatos, Nat. Mat. 2, 382 (2003). 16.
T. Mokari, U. Banin, Chem. Mater. 15, 3955 (2003). 17. E. Nahum et
al., Nano Lett. 4, 103 (2004).
The above references will be acknowledged in the text below by
indicating their numbers [in brackets] from the above list.
BACKGROUND OF THE INVENTION
Anisotropic growth of nanomaterials has led to the development of
complex and diverse nano-structures such as rods, tetrapods,
prisms, cubes and additional shapes. These architectures display
new properties and enrich the selection of nano-building blocks for
electrical, optical and sensorial device construction. Even greater
complexity and new function is introduced into the nanostructure by
anisotropic growth with compositional variations. This has been
elegantly realized by growing semiconductor heterostructures such
as p-n junctions and material junctions in nanowires [1,2], and in
the case of colloidal nanocrystals, in growth of rodlike CdSe/CdS
core-shell particles [3] from spherical CdSe core nanocrystals and
in complex branched growth.
A process for the preparation of nanocrystalline semiconductors,
having rod-like shape of controlled dimensions is described in U.S.
Pat. No. 5,505,928 [13] and in WO 03/097904 [4] for especially
Group III-V semiconductors, [4]. Nanocrystal particles having core
with first crystal structure, and at least one arm with second
crystal structure are described in WO 03/054953 [5].
Recently there have been several reports relating to connectivity
formation for micron-long quasi-one-dimensional structures such as
nanotubes and nanowires [6, 7, 8]. However, wiring of shorter
semiconductor nanoparticles such as rods and tetrapods, with arm
lengths of less then 100 nm has not been achieved yet.
SUMMARY OF THE INVENTION
There is a need in the art for new nanostructures having selective,
well-defined anchor points grown upon them for use in self-assembly
in solution and onto substrates and for electrical or chemical
connectivity. Such nanostructures and method for their manufacture
are not available to date.
Examples of desired nanostructures with well-defined anchor points
would be nanoparticles having elongated shapes with different tips
e.g. metal tips, grown onto at least one end portion thereof, in a
controllable and repeatable manner that would also provide an
electrical and/or chemical contact point. The different zones grown
onto the nanoparticles end portion would provide well-defined
anchor points onto which selective chemistries could be used to
generate self-assembled structures of controlled arrangements.
The present invention thus provides in a first aspect nanostructure
having at least one elongated structure element comprising a first
material, where the elongated structure element bears on at least
one of its end portions a second material that differs from the
first material in at least one property selected from: electrical
conductivity, chemical reactivity (i.e. the property of binding,
where "binding" refers to covalent binding, electrostatic
interaction or other close association between molecules) and
composition.
The first material may be made of metal, semiconductor or
insulating material or mixtures thereof. The second material
comprises of metal or metal alloy. In such case the second material
differs from the first material in electrical conductivity and/or
chemical reactivity and/or composition. Alternatively, the second
material comprises conductive polymer, semiconductor, insulating
material or mixtures thereof. In such case the second material
differs from the first material in at least one property selected
from electrical conductivity, chemical reactivity and
composition.
In a preferred embodiment the present invention provides new
nanoscale structures in which a metal or metal alloy tip
(conductive zone) is present on at least one of the end portions of
the nanostructure. The novel nanostructures have an elongated shape
such as wire, tube, rod, bipod, tripod and tetrapod. The
nanostructures of the present invention differ from the nanotubes
and nanowires bearing electrodes formed by evaporation, described
in references [6-8] above since the electrodes of the prior art
were formed on some region of the tube or wire and not on their
tips.
Preferably, the nanostructures of the invention have at least one
elongated structure element comprising a first material, where said
elongated structure element bears on at least one end portion
thereof a second material selected from metal and metal alloy. The
first and second materials differ in their electrical conductivity
and/or chemical reactivity and/or composition.
The term "material" in the context of the present invention relates
to any solid substance of which the nanostructures of the invention
are made, that may be composed of one or more elements or
ingredients. Alloys, composites, layered structures and matter
formed by chemical union of two or more elements or ingredients are
also within the scope of this definition.
The first material mentioned above is selected from semiconductor
material, insulating material, metal and mixtures thereof.
Preferably, the first material is a semiconductor material selected
from Group II-VI semiconductors, Group III-V semiconductors, Group
IV-VI semiconductors, Group IV semiconductors, alloys made of these
semiconductors, combinations of the semiconductors in composite
structures and core/shell structures of the above semiconductors.
Even more preferably, the nanostructures are made from Group II-VI
semiconductors, alloys made from Group II-VI semiconductors and
core/shell structures made from Group II-VI semiconductors.
Specific examples of Group II-VI semiconductors are CdSe, CdS,
CdTe, ZnSe, ZnS, ZnTe alloys thereof, combinations thereof and
core/shell layered-structures thereof.
The first material is different than the second material in at
least one property selected from electrical conductivity, chemical
reactivity and composition. The second material may be a metal,
metal alloy, insulating material, conductive polymer or
semiconductor. Examples of preferred metals are transition metals
such as for example Cu, Ag, Au, Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr, Fe,
Ti and alloys of such metals. Examples of conductive polymers,
including composites and metal doped conductive polymers, are
polyaniline, polypyrrole, polythiophene, composites thereof or
doped versions thereof. Examples of semiconductors for use as the
second material are Group II-VI semiconductors, Group III-V
semiconductors, Group IV-VI semiconductors, Group IV
semiconductors, alloys made of these semiconductors and
combinations of the semiconductors in composite structures.
For nanostructures composed of semiconductor for both first and
second materials, such that an elongated nanostructure is made of a
first semiconductor that has formed on at least one of its end
portions a zone made of a second semiconductor, in such case the
second semiconductor material is different than the first
semiconductor in at least one of the following properties:
electrical conductivity, chemical reactivity, band gap and
composition. This kind of nanostructures of the present invention
are different from those described in Reference [3] where CdSe dots
are surrounded by a CdS shell that is elongated in one direction.
The nanostructures described in reference [3] are also grown in a
completely different method starting from the dot and growing the
shell around it which is then elongated. Examples of suitable
semiconductor materials for use as the first or second material are
herein described.
The present invention provides, in another of its aspects, a method
for forming a zone on at least one end portion of a nanostructure,
wherein that zone differs from the whole nanostructure, the method
comprising: contacting a solution comprising nanostructures
composed of at least one elongated structure element, with a
solution comprising an agent selected from metal source, metal
alloy source, conductive polymer source, insulating material source
(e.g. such as oxides and organic materials) and semiconductor
source, to obtain upon isolation nanostructures bearing at least
one zone on said at least one elongated structure thereof that
differs from the nanostructure in at least one property selected
from: electrical conductivity, chemical reactivity and
composition.
In a preferred embodiment, the method of the invention comprises:
contacting a solution comprising nanostructures composed of at
least one elongated structure element, with a solution comprising
metal source or metal alloy source, to obtain upon isolation
nanostructures bearing at least one zone comprising metal or metal
alloy on said at least one elongated structure thereof, where the
metal or metal alloy zone differs from the whole elongated element
in electrical conductivity and/or chemical reactivity and/or
composition.
The method is carried out in solution, at a temperature between
about -40.degree. C. to about 400.degree. C., preferably between
about 10.degree. C. to about 80.degree. C., more preferably between
about 20.degree. C. to about 30.degree. C. and even more preferably
at room temperature.
According to a preferred embodiment the reaction is carried out in
the presence of at least one of the following agents in addition to
said nanostructures and metal source of conductive second material:
electron donor, surfactant and stabilizer. In a specific embodiment
the same agent may function as electron donor, and/or surfactant
and/or stabilizer.
The nanostructures used in the method of the invention have an
elongated shape, for example of rods, wires, tubes, or in branched
form. More preferably the nanostructures have an elongated shape
such as for example nanorods and branched shape such as bipods,
tripods, tetrapods and the like. The term "nanorod" or "rod" as
used herein is meant to describe a nanoparticle with extended
growth along the first axis while maintaining the very small
dimensions of the other two axes, resulting in the growth of a
rod-like shaped nanocrystal of very small diameter in the range of
about 1 nm to about 100 nm, where the dimensions along the first
axis may range from about several nm to about 1 micrometer. The
term "tetrapod" is meant to describe a shape having a core from
which four arms are protruding at tetrahedral angles. In the case
of nanorods, the resulting structures after treating them with the
metal or metal alloy source are shaped as "nano-dumbbells".
The nanostructures have an elongated shape or even a branched shape
and serve as a template at the nanometer level for the deposition
of a conducting material, and as it will be described and
exemplified herein below, the deposition is accomplished in a
controllable manner on at least one end portion of the elongated
elements of the nanostructures.
The nanostructures are made of a first material comprising
semiconductor material, insulating material, or mixtures thereof.
Preferably, the nanostructures are made of semiconductor material
selected from Group II-VI semiconductors, such as for example CdS,
CdTe, ZnS, ZnSe, ZnO, HgS, HgSe, HgTe and alloys (e.g. CdZnSe);
Group III-V semiconductors such as InAs, InP, GaAs, GaP, InN, GaN,
InSb, GaSb, AlP, AlAs, AlSb and alloys (e.g. InAsP, CdSeTe, ZnCdSe,
InGaAs and the like); Group IV-VI semiconductors such as PbSe, PbTe
and PbS and alloys; and Group IV semiconductors such as Si and Ge
and alloys. Additionally, combinations of the above in composite
structures consisting of sections with different semiconductor
materials, for example CdSe/CdS or any other combinations, as well
as core/shell structures of different semiconductors such as for
example CdSe/ZnS core/shell nanorods, are also within the scope of
the present invention.
The nanostructures may also be made of an insulating material such
as for example oxides and organic materials or, alternatively the
nanostructures are made of metals. Examples of oxides are silicon
oxide, titanium dioxide, zirconia. Metals include Au, Ag, Cu, Pt,
Co, Ni, Mn and the like, and various combinations and alloys
thereof. Organic materials suitable for use in the nanostructures
are for example polymers.
The metal or metal alloy source used in the method of the present
invention for the fabrication of the conductive zone at the end
portion of an elongated structure element preferably comprises a
transition metal or mixture of such metals. A variety of metals may
be used. This includes noble metals such as Cu, Ag, Au, or other
transition metal elements such as Pt, Co, Pd, Ni, Ru, Rh, Mn, Cr,
Fe, Ti and the like. The metal growth procedure is done by using a
proper metal salt source, for example AuCl.sub.3 for Au growth,
Ag(CH.sub.3COO) for silver growth, Cu(CH.sub.3COO).sub.2 for Cu
growth, PtCl.sub.4 or Pt(acetylacetonate) for Pt growth,
Ni(cyclooctadiene).sub.2 for Ni growth, CO.sub.2(CO).sub.8 or
CoCl.sub.2 for Co growth, and Pd(NO.sub.3).sub.2 for Pd growth.
The metal salts are dissolved in a proper organic solvent such as
hydrocarbons, e.g. hexanes, cyclohexanes, etc., aromatic solvents
e.g. toluene, etc., using a proper surfactant and/or stabilizer
that stabilizes the nanostructures and the metal salt by preventing
aggregation. The organic solvent used in the method of the present
invention is one capable to solubilize both the nanostructure and
the metal source.
Examples of surfactants are cationic surfactants such as ammonium
salts, alkyl pyridinium and quaternary ammonium salts. More
specific examples are tetrabutylammonium borohydride (TBAB),
di-dodecyldimethylammonium bromide (DDAB), cetyltrimethylammonium
bromide (CTAB), and salts of quaternary ammonium with acetate group
ions such as acetate group ions, pivalate, glycolate, lactate and
the like.
Stabilizer compounds used in the method of the invention are such
compounds capable to coordinate to the nanostructure surface and/or
the metal particle surface and hence prevent aggregation of the
nanostructures. Examples of stabilizers are aliphatic amines, e.g.
hexadecylamine, dodecylamine, octylamine, alkylthiols, e.g. hexane
thiol, decylthiol, dodecylthiol, etc. and carboxylic stabilizers
such as myristic acid, palmitic acid and citrate.
The metal or metal alloy salt is first dissolved in an organic
solvent comprising a surfactant and a stabilizer to give a solution
which is subsequently added in a controllable manner and a suitable
temperature to the nanostructures solution.
When an electron source is desired in the method of the invention,
an electron donor compound may be used. Examples of electron donors
are organic compounds, such as aliphatic amines, hydrides such as
sodium borohydride and the like, ascorbic acid and other reducing
agents. A stabilizer can also serve as an electron donor, for
example an aliphatic amine. A surfactant can also serve as an
electron donor, for example tetrabutylammonium borohydride (TBAB).
According to another example the electron source is obtained from
an electron beam device. Alternatively, one may use electromagnetic
radiation such as that from a light source including a lamp or
laser in order to excite the nanoparticles or the metal source.
In another preferred embodiment, the present invention provides a
method for forming in solution medium an electrically conductive
zone on a nanostructure having at least one elongated structure,
the method comprising: contacting an organic solution comprising
semiconductor nanostructures composed of at least one elongated
structure element, with an organic solution comprising a metal or
metal alloy source, a stabilizer and/or surfactant to obtain upon
precipitation semiconductor nanostructures bearing at least one
electrically conductive zone on said at least one elongated
structure thereof. Preferably, the nanostructures used in the
method of the invention are in the form of nanorods, tetrapods or
any other branched structure and are made of elements of Group
II-VI, alloys of such elements or core-shell layered structures
thereof.
The method of the present invention provides new functionalities to
the nanostructures, the most important of which are the formation
of reactive anchor points for directed self assembly and for
electrical or chemical connectivity. The selective tip growth of
metal contacts provides the route to an effective wiring scheme for
soluble and chemically processable nanostructures with branched
shapes. This would allow to fully realize the potential of
miniaturization of devices using such nano-building blocks, while
employing the powerful principles of self-assembly to connect them
to the `outside` world.
Nanostructures obtainable by the method of the invention are within
the scope of the invention.
Electronic and optical devices comprising the nanostructure of the
invention or into which the nanostructure of the invention is
integrated are also within the scope of the invention. Non-limiting
examples are electrodes, transistors e.g. field effect transistor,
memory devices, and the like and self assembled constructs
comprising a plurality of nanostructures, wherein each
nanostructure is linked to another nanostructure in the construct
through its conductive zone.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to understand the invention and to see how it may be
carried out in practice, preferred embodiments will now be
described, by way of non-limiting example only, with reference to
the accompanying drawings, in which:
FIG. 1 illustrates TEM (Transmission Electron Microscopy) images
A-D showing growth of Au onto CdSe quantum rods of dimensions
29.times.4 nm (length.times.diameter). FIG. 1A shows the rods
before Au growth, FIG. 1B shows selective Au growth of about 2.2
nm; FIG. 1C shows selective Au growth of about 2.9 nm; and FIG. 1D
shows selective Au growth of about 4 nm.
FIG. 2A illustrates the EDS (Energy dispersive X-ray spectroscopy)
spectrum of a gilded CdSe rod sample. The relative atom percentage
of Au:Cd:Se is 18%:42%:40%.
FIG. 2B illustrates a powder X-ray diffraction comparing CdSe rods
before (1), and after (2) Au growth.
FIGS. 2C and D illustrate HRTEM images of a single nano-dumbbell
and a nano-dumbbell tip, respectively. The CdSe lattice for the rod
in the center, and Au tips at the rod end portions, can be
identified as marked.
FIG. 3 illustrates TEM images A-H showing growth of Au on tips of
various CdSe quantum rods and CdSe tetrapods. FIGS. 3A and 3B:
12.times.4 nm quantum rods before and after Au growth,
respectively. FIGS. 3C and 3D: 29.times.4 nm quantum rods before
and after Au growth, respectively. FIGS. 3E and 3F: 60.times.6 nm
quantum rods before and after Au growth, respectively. Au growth on
CdSe tetrapods showing a general view is presented in FIG. 3G and
higher magnification image for one tetrapod is presented in FIG.
3H.
FIG. 4A illustrates the absorption spectra for CdSe/ZnS core/shell
nanorod sample with varied Au tip size compared to the original rod
template, where Au tip size is indicated for each trace. Spectra
are offset vertically for clarity. Inset shows TEM image of the
sample after Au growth (scale bar is 50 nm).
FIG. 4B illustrates the photoluminescence (PL) spectra for CdSe/ZnS
core/shell nanorod sample with varied Au tip size compared to the
original rod template, where Au tip size is indicated for each
trace. Traces were multiplied by 25, 50 and 50 for the 2 nm, 3.2
nm, and 4.5 nm Au tips, respectively, for clarity. Inset shows a
plot of relative PL yield for template (.PHI..sub.0) over Au-rod
(.PHI.), versus Au ball size. Measurements were performed for rod
solutions in a sealed cuvette under Ar using the 454 nm line of an
Ar-ion laser with intensity of 100 mW. Fluorescence was collected
using identical conditions for all solutions in a right angle
configuration with a spectrograph/CCD setup, with 500 ms
integration time.
FIG. 5 shows sizing histograms for gilded rods shown in FIG. 1.
Histograms for rod diameter (FIG. 5A), length (FIG. 5B) and Au
diameter (FIG. 5C) are shown for the four samples: 1. Original
29.times.4 nm rods, 2. Rods (10 mg) after treatment with 4 mg
AuCl.sub.3, 25 mg DDAB and 40 mg dodecylamine. 3. Rods (10 mg)
after treatment with 8 mg AuCl.sub.3, 50 mg DDAB and 90 mg
dodecylamine. 4. Rods (10 mg) after treatment with 13.5 mg
AuCl.sub.3, 100 mg DDAB and 160 mg dodecylamine.
FIG. 6 illustrates TEM of product from mixture of rods with
AuCl.sub.3 and DDAB without dodecylamine, FIG. 6A. before exposure
to the TEM electron beam--aggregated rods are seen. FIG. 6B. after
exposure to the TEM electron beam--Au patches appear on the
rods.
FIG. 7 shows conductive atomic force microscopy current image of
single nano-dumbbell measured at a sample bias of 2V. Higher
conductance through the Au tips is observed, as seen also by the
current cut taken along the rod (along plotted line). In the inset,
a TEM image with the same scale of a Au-rod is shown for
comparison.
FIG. 8 shows the self assembly of nano-dumbbells into chains formed
by adding hexane dithiol bifunctional linker to a solution of
nano-dumbbells.
FIG. 9 illustrates TEM images A-D showing one-sided growth of Au on
tips of CdSe quantum rods having different sizes: FIG.
9A--18.times.3.5 nm; FIG. 9B--25.times.4 nm; FIG. 9C--50.times.4
nm; FIG. 9D--the same as (C) with different gold tip size.
DETAILED DESCRIPTION OF THE INVENTION
The method is exemplified hereinbelow with reference to selective
growth of metal tips onto semiconductor nanorods and tetrapods.
In a method for selective growth of contacts made of gold,
AuCl.sub.3 was dissolved in toluene by use of
di-dodecyldimethylammonium bromide (DDAB) and dodecylamine, and the
resulting solution was added to a toluene solution comprising of
colloidal grown nanorods or tetrapods. The method is exemplified
for the prototypical CdSe nanocrystal system that is highly
developed synthetically and widely studied for its size and shape
dependent properties.
CdSe rods and tetrapods of different dimensions (see below), were
prepared by high temperature pyrolisys of suitable precursors, in a
coordinating solvent containing a mixture of trioctylphosphineoxide
(TOPO), and of phosphonic acids [9]. In a typical Au growth
reaction, a gold solution was prepared containing 12 mg AuCl.sub.3
(0.04 mmol), 40 mg of DDAB (0.08 mmol) and 70 mg (0.37 mmol) of
dodecylamine in 3 ml of toluene and sonicated for 5 minutes at room
temperature. The solution changed color from dark orange to light
yellow. 20 mg (1.times.10.sup.-8 moles of rods, where each rod has
about 7000 CdSe units) of CdSe quantum rods with 29.times.4 nm
size, were dissolved in 4 ml toluene in a three neck flask under
argon. The gold solution was added drop-wise over a period of three
minutes. During the addition, carried out at room temperature, the
color gradually changed to dark brown. Following the reaction, the
rods were precipitated by addition of methanol and separated by
centrifugation. The purified product could then be redissolved in
toluene for further studies.
FIG. 1 presents transmission electron microscopy (TEM) images
showing growth of Au onto CdSe quantum rods of dimensions
29.times.4 nm (length.times.diameter). FIG. 1A shows the rods
before Au growth, while in FIGS. 1B-D, selective Au growth onto the
rod tips is clearly identified as the appearance of points with
enhanced contrast afforded by the higher electron density of the Au
compared with CdSe. The rods now appear as `nano-dumbbells`.
Moreover, by controlling the amount of initial Au precursor, it is
possible to control the size of the Au tips on the nano-dumbbell
end portions, from .about.2.2 nm in FIG. 1B, to .about.2.9 nm in
FIG. 1C, to .about.4.0 nm in FIG. 1D as summarized in Table 1. The
procedure clearly leads to the growth of natural contact points on
the tips of the rods.
TABLE-US-00001 TABLE 1 NC's.sup.1 DDA.sup.2 DDAB.sup.3 AuCl.sub.3
Rods size Gold amount amount amount amount (L .times. D) ball
Sample (mg) (mg) (mg) (mg) nm size (nm) 1 -- -- -- -- 29 .times. 4
(original nm rod) 2 10 mg 40 mg 25 mg 4 mg 25.6 .times. 3.3 2.22 nm
nm 3 10 mg 90 mg 50 mg 8 mg 23.9 .times. 3.4 2.9 nm nm 4 10 mg 160
mg 100 mg 13.5 mg 20.8 .times. 3.2 4 nm nm .sup.1NC--nanocrystals
.sup.2DDA--dodecylamine .sup.3DDAB--didodecyldimethylamonium
bromide
An additional observation from the analysis of .about.200 particles
per sample is that the overall rod length becomes shorter upon Au
growth, and there is also a decrease in the average diameter of the
rods, (Table 1 and FIG. 5 for the complete sizing histograms).
Control experiments with the DDAB and dodecylamine without
AuCl.sub.3 were carried out and also in that case the average rod
dimensions decreased, implying that reduction of rod size is
perhaps related to dissolution of rods in the presence of DDAB and
not to the Au growth.
Several structural and chemical characterization methods have been
carried out in order to verify the material content and structure
of the gold on the rod tips. FIG. 2A shows EDS analysis of a micron
area of rods after growth and the appearance of Au in the gilded
(i.e. goldenized) and purified rod sample is clear. The powder
Xray-diffraction pattern for the 29.times.4 nm rod sample comparing
the rods before and after gold growth is shown in FIG. 2B. The
appearance of the Au (111), (200) and (220) peaks following Au
growth is evident, demonstrating crystalline Au is formed on the
tips.
Further evidence for Au growth onto single rods, is provided by
HRTEM (high resolution TEM) studies of the nano-dumbbells. FIG. 2C
shows a HRTEM image of a single rod after gold treatment. The
lattice image for the rod part composed of CdSe corresponds to
growth of rods along the CdSe <001> axis. The Au is discerned
once again as the region at the end portion with enhanced contrast
and the gold lattice is also shown in FIG. 2D.
Relating to the interface at the Au--CdSe, it is suggested that
Au--Se bonds are formed, analogous to the known AuSe material [10].
This means that the interface is formed with covalent chemical
bonds between the metal and the semiconductor and hence can be
expected to provide good electrical connectivity.
The method for selective Au growth could be easily expanded and
applied to rods of arbitrary dimensions, and to tetrapods, as well
as to growth of other metals and to rods made of various
semiconductor materials.
FIG. 3 shows TEM images for three rod samples of dimensions
12.times.4 nm (FIG. 3A, B), 29.times.4 nm (FIG. 3C, D), and
60.times.6 nm (FIG. 3E, F), before and after Au treatment. The
presence of the high-contrast tips on the treated rods, forming
nano-dumbbells, is evident in all cases. Highly selective tip
growth is discerned and demonstrated for three rod sizes and could
easily be applied to arbitrary rod sizes. In addition the method
was applied to a CdSe tetrapod sample, as can be seen in FIG. 2G
showing several tetrapods, and in FIG. 2H showing an enlargement of
one tetrapod, following the Au growth process. In this case, the
growth occurs selectively on all the tips of the tetrapods leading
to a tetrahedral arrangement for the Au tips, and once again
providing the natural contact points for this unique structure, for
further self-assembly and for electrical connections.
In another example, CdTe nanostructures served as the template for
growing various metals on its end portions. The synthesis of the
CdTe in different shapes is known [11,14]. In a typical synthesis
of CdTe rods, a mixture of 1 mmol of CdO dissolved in 1.125 gr
oleic acid and 2.5 gr of 1-octadecene is heated in three neck flask
to 300.degree. C. to obtain a clear colorless solution. In the
glove box, a solution of Te (0.5 mmol of Te is dissolved in 1 ml of
TOP) is prepared and brought out in a vial sealed with septum to
the injection. After the injection of the Te solution into the
mixture in the three necked flask, the mixture is cooled to
260.degree. C. for growth. Modification of this procedure in terms
of the temperature or precursor concentration results in size and
shape changes. The oleic acid is used as a ligand and it dissolves
the CdO in the octadecene.
Another specific semiconductor material that may be used is CdS,
which is controllable in size and shape. The synthesis is based on
the same principle which is injection precursor to hot solution,
the Cd and S precursor in this case is Cd(S.sub.2CNEt).sub.2 [12].
In typical synthesis of CdS nanorods, a warm solution of
Cd(S.sub.2CNEt).sub.2 (50 mg dissolved in about 0.3 g of
hexadecylamine (HDA) at about 70.degree. C.) is injected into hot
solution of HDA and after 1 hr is cooled to 70.degree. C. and
treated with ethanol and separated by centrifuging. Controlling the
shape of the nanocrystals is done by changing the growth
temperature of the synthesis from 300.degree. C. (rods) to
120.degree. C. (tetrapods).
Metal tips by the method described above have also been grown onto
CdSe/ZnS core/shell nanorods (29.times.4 nm rods with 2 monolayer
ZnS shell) with initial emission quantum yield of 2% [15].
Treatment of these rods with DDAB and dodecylamine without Au led
to an increased quantum yield of 4%, likely because of the effect
of the excess amine. Several Au sizes were grown from about 2 nm to
about 4.5 nm Au at the tips of the rods.
The metalized structures (in the case of Au growth the formed
structures are termed herein "gilded structures") exhibit new and
different electronic, electrical and optical properties as compared
to the original rods, due to the strong effect of the metal on the
semiconductor properties. Absorption and photoluminescence (PL)
measurements were carried out to study the effect of Au growth on
the rod optical properties as shown in FIG. 4. Absorption spectra
(FIG. 4A) for the small Au tips on the rods still shows the
excitonic structure but with increased absorbance in the visible
and the appearance of a tail to the red of the particle gap. Upon
increased Au size, the features of the absorption of the rods are
washed out and the tail to the red becomes more prominent. The
spectra should contain in principle contributions from the
semiconductor part and the plasmon resonance associated with the Au
tips. However, attempts to add spectra of the rod template and Au
nanocrystals did not reproduce the observed absorption and we
suggest that the spectra are not a simple sum of both components
because of the modified electronic structure of the Au-rod
nano-dumbbell system. The strong mixing of the semiconductor and
metal electronic states leads to modified density of states
exhibiting broadened levels and a reduced band-gap.
The significant coupling of the Au is also observed for the PL
(FIG. 4B) that undergoes considerable quenching with increased Au
ball size, by a factor of about 100 initially for the smaller Au
balls (about 2 nm), and gradually down to a factor of about 500 for
the large Au balls (about 4.5 nm). Quenching of the emission by the
metal end portions is expected via the new non-radiative pathways
created by the proximity of metals, likely leading to electron
transfer to the Au. Moreover, a systematic dependence of quenching
on Au size is seen as shown in the Stern-Volmer type plot (inset of
FIG. 4B). Both absorption and emission spectra exemplify the
significant effect of the Au on the semiconductor rod properties in
this new system, further proving the strong bonding of the Au to
the CdSe rod.
The selective tip growth of Au onto the rods and tetrapods not only
provides important developments for enabling electrical
connectivity and new paths for self-assembly for such
nanostructures. It is also an interesting and novel chemical
reaction route with clear selectivity and anisotropic character.
The reaction mechanism for the gold growth entails a reduction of
Au. Examining by TEM the Au solution with DDAB and dodecylamine,
already reveals the formation of Au particles. When the reaction is
carried out without dodecylamine, considerable aggregation of the
CdSe rods was seen (FIG. 6A). Additionally, without the amine,
growth of Au on rods was not apparent initially and only after the
irradiation under the electron beam of the TEM we could observe
some Au growth (FIG. 6B).
One of the benefits of the method of the present invention is its
specificity leading to selective tip growth. This results from the
preferential adsorption of the metal, e.g. Au complex formed in the
Au solution by adding Au salt to DDAB and dodecylamine onto the
nanostructures end portions. The tips are more reactive due to the
increased surface energy and also possibly due to imperfect
passivation of the ligands on these faces, which also leads to
preferential growth along the <001> axis of CdSe rods. Once
Au nucleates on the end portion, it is preferential for additional
Au to adhere and grow on that seed. This gains support from
controlling the extent of Au growth on the rod tips by using
increased concentration of Au in the gold solution as was shown in
FIG. 1. Moreover, early Au growth as shown in FIG. 1B reveals that
in some rods preferential early growth occurs on one tip, in
agreement with the surfactant-controlled growth model suggested for
CdSe rods [9].
Under different conditions it is also possible to achieve growth of
the metal only on part of the end portions of the nanostructure.
Thus, for example, when the nanostructure is in the form of rod, it
is possible under suitable conditions to perform metallization only
on one end out of the two end portions of the rod forming so called
`monobells`. This performance is exemplified in FIG. 9 showing one
sided growth of Au on CdSe nanorods of various dimensions: FIG.
9A--18.times.3.5 nm; FIG. 9B--25.times.4 nm; FIG. 9C--50.times.4
nm; FIG. 9D--the same as (C) with different gold tip size.
The route for one-sided growth involves using higher concentration
of Au precursor. In a typical one sided Au growth reaction, a gold
solution was prepared containing 2.5 mg AuCl.sub.3 (0.008 mmol), 20
mg of DDAB (0.04 mmol) and 35 mg (0.185 mmol) of dodecylamine in 4
ml of toluene and sonicated for 5 minutes at room temperature. The
solution changed color from dark orange to light yellow. 0.8 mg
(2.35.times.10.sup.-10 moles of rods) of CdSe quantum rods with the
size 50.times.4 nm were dissolved in 20 ml toluene in a three neck
flask under argon. In the case of 25.times.4 nm rods size, 0.9 mg
(4.8.times.10.sup.-10 moles of rods) were dissolved in 20 ml
toluene with keeping the same gold solution amounts. The gold
solution was added drop-wise over a period of three minutes. During
the addition, carried out at room temperature, the color gradually
changed to dark brown. In the monobells synthesis (one sided
growth) as mentioned here, the mole ratio of Au to nanorods is
significantly higher as compared with the nano-dumbbell (two sided
growth) case. One sided growth is desirable for some self-assembly
schemes and also for different non-linear optical properties such
structures may reveal in optical applications. Additionally, a one
sided grown structure has intrinsic asymmetry and can serve as a
diode element.
It is important to note that in some cases Au growth was identified
on branching and defect points, but at slower rate compared to the
distinctive tip growth discussed above. This can be seen in FIGS.
3E and 3G, where weak dark Au spots appear also in some positions
other then the tips of the long rods and tetrapods. This growth can
be controlled by the amounts of Au added to the rods. At such
defect points, such as points where the diameter of the structure
changes, there is also increased reactivity due to the imperfect
chemical bonding and increased surface energy. This leads to Au
adhesion and growth in agreement with the mechanism for tip growth.
It is emphasized that the tip growth occurs more readily and
rapidly then growth on the defects and hence can be controlled to
achieve contact points.
The method may easily be expanded to additional semiconductor
nanocrystal systems and to additional metals, to tailor the metal
tip contacts as desired and the semiconductor element as well.
One application for the metal tips is in serving as electrical
contact points. The role to be played by the Au tips as contact
points for wiring the rods is exemplified by conductive atomic
force microscopy (C-AFM) measurements carried out on gilded
60.times.6 nm rods. Rods were deposited onto a conducting highly
ordered pyrrolitic graphite substrate, and embedded in a thin layer
of poly methyl methacrylate (PMMA) to avoid dragging by the tip as
reported earlier for regular rods [16]. The current image of a
single rod measured by this method reveals that already at a bias
of 1.5-2 V, small tunneling current is flowing through the tips
which are composed of Au, while the central part of the rod
consisting of the semiconductor is non-conductive at these
conditions (see FIG. 7). The small tunneling current is determined
by the tunneling barriers at tip-nanocrystal and
nanocrystal-substrate junctions, dominated primarily by the PMMA.
This measurement reveals the significantly higher conductance of
the Au tips which would be critical for using them as electrical
contact points.
Several strategies can be employed to realize such contacts. It is
possible to form the metallized nanorods or other branched
structures onto a substrate, identify their position, and then
write by electron-beam lithography electrodes to overlap with the
Au tips. In a different approach, it is also possible to deposit
the metallized rods onto pre-existing electrode structures, with or
without electrostatic trapping by an applied electric field. Since
the metal tipped nanostructures enable the connectivity to
electrode structures, this clearly opens the path for using them as
transistors, in sensing applications, and in light emitting or
light detecting devices.
The metal end portions can also impart the rods with advantageous
and novel optical properties. They exhibit enhanced linear and
non-linear optical properties. The polarizability of such a
structure may obviously be significantly increased compared with
that of the regular rods. For example, enhancement in second, third
and higher harmonic generation and also the observation of novel
plasmon resonances related to highly controlled distances that
could be tailored for the metal tips on rods.
Additionally, is possible to apply the powerful approach of self
assembly by using for example, biological templates e.g. DNA, for
creating the connections to the metal tips of nanorods or of
branched structures, or bifunctional ligands such as dithiols or
diamines for binding preferentially to the Au tips. In such
applications the metal tips serve as selective anchor points for
ligands and chemistries preferential for the Au surface. Such self
assembly could for example be done in solution or onto surfaces. In
solution, examples include formation of AAAA chains where A
represents rods of one type. This is done by adding bifunctional
ligands such as dithiols, for example hexane dithiol, to a solution
with gilded nanorods. The preferential binding of thiols to the Au
tips leads to chain formation as can be seen in FIG. 8. Another
example is the formation of ABAB chains where A represents one rod
type and B another rod type. Here biochemical linkers such as
avidin-biotin chemistry or DNA linking can be used to make
selective ABAB chains. In another approach, combining tetrapods
with rods on tip to tip basis may lead to formation of propeller
structures.
The same chemistries can be used to self-assemble rods and
tetrapods with Au tips onto patterned or non-patterned substrates.
For example, a gold or silicon substrate is used together with a
bifunctional ligand that binds with one function to the substrate
and with the second function to the Au tip on the
nanostructure.
Metal tipped structures also provide selective metal growth points
for additional materials via a seeded growth solution-liquid-solid
mechanism.
* * * * *